Equal-path interferometer

Abstract

An optical assembly for use in an interferometer is provided. The optical assembly includes first and second partially reflective surfaces positioned along an optical axis and oriented at different non-normal angles to the optical axis. The second partially reflective surface is configured to receive light transmitted through the first partially reflective surface along the optical path, transmit a portion of the received light to a test object to define measurement light for the interferometer and reflect another portion of the received light back towards the first partially reflective surface to define reference light for the interferometer. The reference light makes at least one round trip path between the second and first partially reflective surfaces.

1. An optical assembly for use in an interferometer, the optical assembly comprising:
 first and second partially reflective surfaces positioned along an optical axis and oriented at different non-normal angles to the optical axis, 
 wherein the second partially reflective surface is configured to: 
 i) receive light transmitted through the first partially reflective surface along the optical path; 
 ii) transmit a portion of the received light to a test object to define measurement light for the interferometer; and 
 iii) reflect another portion of the received light back towards the first partially reflective surface to define reference light for the interferometer, wherein the reference light makes at least one round trip path between the second and first partially reflective surfaces. 
 
     
     
       2. The optical assembly of  claim 1 , wherein the non-normal angles cause the reference light to pass between the first and second partially reflective surfaces at least one time before the second partially reflective surface reflects the reference light back along the optical axis. 
     
     
       3. The optical assembly of  claim 2 , wherein the non-normal angles cause the reference light to contact one of the partially reflective surfaces at normal incidence during one of the passes there between. 
     
     
       4. The optical assembly of  claim 1 , wherein the non-normal angle for the first partially reflective surface is two times the non-normal angle for the second partially reflective surface. 
     
     
       5. The optical assembly of  claim 1 , wherein the non-normal angle for the first partially reflective surface is one and a half times the non-normal angle for the second partially reflective surface. 
     
     
       6. The optical assembly of  claim 1 , wherein the second partially reflective surface is configured to combine the measurement light, after the measurement light reflects from the test object back to the second partially reflective surface, with the reference light, after the reference light makes the at least one round trip between the second and first partially reflective surfaces. 
     
     
       7. The optical assembly of  claim 1 , comprising a first optical element having the first partially reflective surface; and a second optical element having the second partially reflective surface. 
     
     
       8. The optical assembly of  claim 7 , wherein the first and second optical elements each has another surface having an anti-reflection coating. 
     
     
       9. The optical assembly of  claim 7 , wherein the first partially reflective surface is spaced away from the second partially reflective surface at a distance that is greater than a depth of focus of an imaging module that captures an interference pattern between the reference light and the measurement light. 
     
     
       10. The optical assembly of  claim 9 , wherein optical elements of the interferometer are positioned such that the reference light does not pass through glass within the depth of focus of the imaging module. 
     
     
       11. The optical assembly of  claim 7 , wherein the first optical element has another surface having an anti-reflection coating. 
     
     
       12. The optical assembly of  claim 11 , wherein the first optical element is oriented such that the first partially reflective surface faces towards the second partially reflective surface of the second optical element, and the anti-reflection coating of the first optical element faces away from the second partially reflective surface. 
     
     
       13. The optical assembly of  claim 12 , wherein a distance between the first partially reflective surface and the second partially reflective surface is greater than a depth of focus of an imaging module for capturing an interference pattern between the reference light and the measurement light. 
     
     
       14. The optical assembly of  claim 13 , further comprising a dispersion compensator positioned between the first optical element and the second optical element to compensate for a phase difference between the measurement light and the reference light, the dispersion compensator being positioned closer to the third optical element and outside of the depth of focus of the imaging system. 
     
     
       15. The optical assembly of  claim 11 , wherein the first optical element is oriented such that the first partially reflective surface faces away from the second partially reflective surface of the second optical element, and the anti-reflection coating of the first optical element faces towards the second partially reflective surface. 
     
     
       16. The optical assembly of  claim 7 , wherein the partially reflective surfaces are on outer surfaces of the optical elements respectively. 
     
     
       17. The optical assembly of  claim 7 , wherein the partially reflective surfaces are formed at respective internal interfaces within the optical elements. 
     
     
       18. The optical assembly of  claim 1 , further comprising a third partially reflective surface. 
     
     
       19. The optical assembly of  claim 18 , wherein the third partially reflective surface is configured to:
 i) receive light transmitted through the first partially reflective surface along the optical path; 
 ii) transmit a portion of the received light to the test object to define the measurement light; and 
 iii) reflect another portion of the received light back towards the first partially reflective surface to define a second reference light for the interferometer, wherein the second reference light makes at least one round trip path between the second and first partially reflective surfaces. 
 
     
     
       20. The optical assembly of  claim 1 , further comprising a collimator to receive light from a light source and project collimated light to the first partially reflective surface. 
     
     
       21. The optical assembly of  claim 1 , further comprising a field lens to receive light from a light source and project the light to the first partially reflective surface, the field lens being positioned outside of an imaging path traveled by the reference light after the reference light is reflected by the first partially reflective surface and before the reference light is detected by a detector. 
     
     
       22. The optical assembly of  claim 1 , wherein the first partially reflective surface has a reflectivity in the range of about 10% to about 30%. 
     
     
       23. The optical assembly of  claim 1 , wherein second partially reflective surface has a reflectivity in the range of about 40% to about 60%. 
     
     
       24. The optical assembly of  claim 1  in which the first partially reflective surface comprises a non-planar surface. 
     
     
       25. An interferometry system comprising:
 the optical assembly of  claim 1 ; and 
 an interferometer base comprising a light source and a detector; 
 wherein the light source is configured to produce the light transmitted through the first partially reflective surface and received by the second partially reflective surface, and 
 wherein the detector is configured to receive combined light comprising the measurement light and the reference light and provide information about a spatial distribution of the combined light. 
 
     
     
       26. The interferometry system of  claim 25 , wherein the interferometer base further comprises an aperture stop positioned to block light from the interferometer base that contacts the first partially reflective surface along the optical axis and reflects from the first partially reflective surface back to the interferometer base. 
     
     
       27. The interferometry system of  claim 25 , wherein the interferometer base further comprises an aperture stop positioned to block light from the interferometer base that contacts the first partially reflective surface along the optical axis and reflects from the first partially reflective surface back to the interferometer base. 
     
     
       28. The interferometry system of  claim 25 , further comprising a mount for supporting the test object. 
     
     
       29. The interferometry system of  claim 25 , wherein the mount is positioned to define an optical path length for the measurement light that is substantially equal to an optical path length for the reference light. 
     
     
       30. The interferometry system of  claim 25 , further comprising a phase shifter for varying the optical path length difference between the measurement light and the reference light. 
     
     
       31. The interferometry system of  claim 30 , wherein the phase shifter mechanically couples the interferometer base to the optical assembly and is configured to vary the distance between the optical assembly and the test object to vary the optical path length for the measurement light. 
     
     
       32. The interferometry system of  claim 25 , wherein the source is a broadband source for providing low-coherence interferometry measurements. 
     
     
       33. The interferometry system of  claim 25 , wherein the source is a narrow-band laser source. 
     
     
       34. The interferometry system of  claim 25 , wherein the source is adjustable between a broadband mode for low-coherence interferometry and a laser mode for high-coherence interferometry. 
     
     
       35. The interferometry system of  claim 34 , wherein the source is a laser diode that operates in the broadband mode when driven at a current below its laser threshold and operates in the laser mode when driven at a current above its laser threshold. 
     
     
       36. An interferometry method comprising:
 positioning first and second partially reflective surfaces along an optical axis; 
 orienting the first and second partially reflective surfaces at different non-normal angles relative to the optical axis; 
 transmitting light through the first partially reflective surface along a direction parallel to the optical axis to the second partially reflective surface; 
 at the second partially reflective surface, transmitting a first portion of the light to a test object to define measurement light, and reflecting a second portion of the light back towards the first partially reflective surface to define reference light; and 
 at the first partially reflective surface, reflecting a portion of the second portion of the light towards the second partially reflective surface such that the reference light makes at least one round trip path between the second and first partially reflective surfaces. 
 
     
     
       37. The method of  claim 36 , wherein orienting the first and second partially reflective surfaces comprises orienting the first and second partially reflective surfaces at different non-normal angles to cause the reference light to pass between the first and second partially reflective surfaces at least one time before the second partially reflective surface reflects the reference light back along the optical axis. 
     
     
       38. The method of  claim 36 , wherein orienting the first and second partially reflective surfaces comprises orienting the first and second partially reflective surfaces at different non-normal angles to cause the reference light to contact one of the partially reflective surfaces at normal incidence during one of the passes there between. 
     
     
       39. The method of  claim 36 , comprising, at the second partially reflective surface, combining the measurement light, after it reflects from the test object back to the second partially reflective surface, with the reference light, after it makes the at least one round trip between the second and first partially reflective surfaces. 
     
     
       40. The method of  claim 39 , comprising providing information about a spatial distribution of the combined light. 
     
     
       41. The method of  claim 36 , comprising providing an aperture stop to block light that is reflected from the first partially reflective surface in a direction away from the second partially reflective surface. 
     
     
       42. The method of  claim 36 , comprising positioning a test object having a reflective surface to define an optical path length for the measurement light that is substantially equal to an optical path length for the reference light. 
     
     
       43. The method of  claim 42 , comprising varying the optical path length difference between the measurement light and the reference light. 
     
     
       44. The method of  claim 43 , comprising varying the distance between an optical assembly and the test object to vary the optical path length for the measurement light, the optical assembly comprising the first and second partially reflective surfaces. 
     
     
       45. The method of  claim 36 , further comprising orienting an optical element having the first partially reflective surface at an outer surface of the optical element such that the outer surface of the optical element having the first partially reflective surface faces towards the second partially reflective surface. 
     
     
       46. The method of  claim 36 , comprising transmitting the reference light from the first partially reflective surface to the second partially reflective surface without passing any glass element. 
     
     
       47. The method of  claim 36 , comprising positioning the second partially reflective surface at a distance away from the first partially reflective surface, the distance being greater than a depth of focus of an imaging module that detects an interference pattern between the measurement light and the reference light. 
     
     
       48. The method of  claim 47 , comprising passing the reference light through a dispersion compensator that compensates a difference in phase between the measurement light and the reference light due to differences in optical path lengths traveled by the reference light and the measurement light, and positioning the dispersion compensator outside of the depth of focus of the imaging module. 
     
     
       49. The method of  claim 36 , further comprising:
 positioning a third reflective surface along the optical axis; 
 orienting the third partially reflective surface to be parallel to the second partially reflective surface; 
 at the third partially reflective surface, transmitting a third portion of the light transmitted by the first partially reflective surface to the test object to define the measurement light, and reflecting a fourth portion of the light back towards the first partially reflective surface to define a second reference light; and 
 at the first partially reflective surface, reflecting a portion of the fourth portion of the light towards the second partially reflective surface such that the second reference light makes at least one round trip path between the second and first partially reflective surfaces. 
 
     
     
       50. The method of  claim 36  in which transmitting light through the first partially reflective surface comprises transmitting collimated light through the first partially reflective surface. 
     
     
       51. The method of  claim 36 , further comprising transmitting the light through a field lens prior to transmitting the light through the first partially reflective surface, and positioning the field lens outside of an imaging path traveled by the reference light after the reference light is reflected by the first partially reflective surface and before the reference light is detected by a detector.